Filter medium



Patented Mar. 27, 1945 FILTER Elbert C. Latin-op and Samuel, 1. Aronovsky, Peoria, 111., assignors to Claude R. Wickard, as Secretary of Agriculture of the United States of America, and his successors in office No Drawing. Application June 26, 1943, Serial No. 492.362

(oi-anus under the m of Mai-en a ias. as amended 'April so, 1928; 370 o. G. 157) Claims.

This application is made under the act of March 3, 1883, as amended by the act of April 30. 1928, and the invention herein described, if patented, may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment to us of any royalty thereon.

This invention relates to filter mediums, and

has for its object the provision of an improved type of filter medium.

The importance of the gas mask in both peacetime pursuits and in war is well known, and constant efforts are being put forth to increase their lightness, compactness, and efllciency. Gas masks consist, essentially, of a gasand vapor-tight headpiece and a filter to remove the noxious fumes or smokes from the air drawn into the mask headpiece by normal breathing. The filter may consist of a plurality of layers, each of which has a specific duty to perform. Thus, there may be an outside porous layer which acts as a support or cover for the inner porous activated material treated to remove the harmful ingredients from the contaminated air. This outer layer must be relatively strong, permeable to air, unaffected by changes in relative humidity of the air, and of light weight. It is also very desirable that this outer layer remove discrete particles or droplets, such as are found in smokes and fumes, from the contaminated air before the latter reaches the activated, absorptive inner layer.

Sheets made from wood alpha pulp and from cotton liner pulp have been used for the outer layer of the gas mask filter, but, although they are satisfactory from the standpoint of permeability to air, they are not suitable for retaining the discrete particles of smokes orfumes except when exceptionally thick and bulky layers are used. This is due to the fact that the meshes formed by the interlacing of such long and relatively thick fibers are large in any oneplane in the sheet and the smoke or fume particles can not be entrapped or retained except by employing numerous planes or layers of the sheet to provide long tortuous paths for these particles. Furthermore, the area of the reactive adsorptive cellulose surfaces of these fibers is relatively small due to the fairly large diameters of these fibers.

It is well known to the art that a mixture of long and short fibers will result in'a more uniform sheet with smaller and more uniform open spaces between the fibers. It is also known that short thin fibers are better suited than short thick fibers for uniform filter material. Asbestos consists of fibers of varying degrees of thickness and the thinnest of these are suitable for admixture with the long relatively thick cellulosic fibers in the production of gas mask filtering media. However, the classification of the asbestos fibers is a rather costly procedure and, as a result, the fibers of the desired size for use in mask filters are quite expensive.

We have foundthat a felted sheet which has properties desired for use in gas mask filters, and

' which is considerably less expensive than the best filter media of the desired properties heretofore known, can be prepared from the relatively short and thin cellulosic fibers of agricultural residues, such as cereal straws, annul grasses. stalks, stems, and weeds, in admixture with the longer cellulosic fibers.

Wood and cotton fibers vary from about 3 to 25 mm. in length and from about 30 to microns in width; whereas, the agricultural residue fibers which we use are about 0.1 to 0.45 mm. long and 5 to 20 microns wide. Furthermore, some of the residue fibers, such as straw fiber, have finely serrated walls, which tend to increase the surface areas available for adsorption.

Another property of the agricultural residue fibers, found to a much lesser extent in wood or cotton fibers, and to a very small extent, if at all, in inorganic fibers such as asbestos, is the increased sorptive activity of the agricultural residue fibers due to their relatively high natural ash contents. This ash, mainly silica and con-' stituting up to 15 percent or more of the entire solid matter of the agricultural residues, is quite uniformly distributed throughout the physical structure of the plant material, and, on proper activation, results in increasing the adsorption surface area. The activation of this structural ash of the agricultural residue fibers is brought about by the alternatealkaline and acid treatments used in the purification of these fibers. We have found that a mixture of wood alpha cellulose fibers and of these agricultural residue alpha cellulose fibers form a sheet with smaller and more uniform pore size without materially impairing the porosity or permeability to air, as compared with a percent woodor cottonfiber sheet, and without increasing the bulk, thickness, or weight-of the sheet. A much larger amount of these agricultural residue fibers can be incorporated into the filter sheet, without im-v pairing the above'desired properties, than is the case with inorganic fine fibers, such as asbestos.

Furthermore, asbestos fiber is several times as heavy as purified cellulose fiber. As an illustration of our invention, we have chosen rice straw and barley straw fibers, two of the finer fibers of small diameter belonging to the cereal straw group which are relatively inexpensive and available in large quantities. Rice straw pulp in particular has a very high adsorptive silica or ash content. Alpha pulp fibers were prepared from these straws and the alpha fibers were mixed in various, proportions with spruce wood alpha pulp or cotton linter pulp and made into hand sheets, using the hand-dip method. The hand sheets were dried and tested with a standard porosity tester for their permeability to air. Mixtures containing up to 80 percent of these agricultural residue alpha pulps were almost as permeable to air "as the 100 percent wood alpha pulp sheets.

Example 1 I Mixtures of wood alpha. pulp with 20, 40, 60, and

80 percent alpha pulp prepared from rice straw were diluted to a consistency of 0.1 percent and mixed thoroughly. Hand sheets of approximate- 1y 90-pound basis weight were obtained by dip ping a mold of the required size. The sheets were then couched from the mold wire, and dried. The dried sheets were tested for porosity by determining the time required for 500 cc. of air to pass through one square inch area of five thicknesses of the sheet. The results show that the rice alpha pulp reduced the porosity of the wood alpha pulp only to a relatively small extent.

The inconsistency in the permeability of the 40 percent rice pulp sample is due to the difilculty of obtaining uniform sheets by this hand method. It is well known to the art that much more uniform formation in the sheet can be obtained on commercial machines than by the hand-dip method.

Example 2 Hand sheets were prepared as in Example 1, except that barley straw alpha pulp was used.

Porosity of Pulp 5 sheets in seconds Wood al ha ul 4.4 p p p barley straw alpha pulp. 4. B barley straw alpha pulp.. 4. 9 barley straw alpha pulp.. 6.3 I barley straw alpha pulp. 6. 6

ception of the filter structure and in conjunction with the above porosity values definitely show the superior beneficial effects of using a mixture of short and thin fibers with the long fibers for the production of a filter medium.

We are not limited to the rice and barley straw products given in the examples set forth above. Any of the cereal straws, annual grasses, stalks, stems, weeds, and similar shortand fine-fibered materials obtained from agricultural residues can be used equally well to prepare the fibers.

Having thus'described our invention, we claim:

1. A filter medium comprising a mixture of short, thin, purified alpha-cellulose fibers prepared from agricultural residues, and long,

purified cellulosic fibers.

2. A filter medium comprising a mixture of short, thin, purified alpha-cellulose fibers of high natural ash content prepared from agricultural residues, and long, purified cellulosic fibers.

3. A filter medium comprising a mixture of short, thin, purified alpha-cellulose fibers prepared from agricultural residues, said fibers having a length of the order of 0.1 to 0.45 millimeter and a width of the order of 5 to 20 microns, and long, purified celluloslc fibers.

4. A filter medium comprising short, thin, purified alpha-cellulose fibers prepared from agricultural residues, said fibers having a length of the order of 0.1 to 0.45 millimeter and a width of the order of 5 to 20 microns, and long, purified cellulosic fibers felted together in sheet form.

5. A filter medium comprising a mixture of short, thin, purified alpha-cellulose fibers prepared from agricultural residues, said fibers having a length of the order of 0.1 to 0.45 millimeter and a width of the order of 5 to 20 microns, and long, purified cellulosic fibers, formed into a loosefill bat.

6. A filter medium comprising a mixture of short, thin, purified alpha-cellulose fibers prepared from cereal straw, said fibers having a length of the order of 0.1 to 0.45 millimeter and a width of the order of 5 to 20 microns, and long, purified cellulosic fibers.

'7. A filter medium comprising a mixture of short, thin, purified rice straw alpha pulp fibers having a length of the order of 0.1 to 0.45 millimeter and a width of the order of 5 to 20 microns, and long, purified celluiosic fibers.

8. A filter medium comprising a mixture of short, thin, purified barley straw alpha pulp fibers having a length of the order of 0.1 to 0.45 millimeter and a width of the order of 5 to 20 microns, and long, purified cellulosic fibers.

9. A filter medium comprising a mixture of short, thin, purified. rice straw alpha pulp fibers having a length of the order of 0.1 to 0.45 millimeter and a width of the order of 5 to 20 microns, and long, purified wood alpha pulp fibers.

10. A filter medium comprising a mixture of short, thin, purified barley straw alpha pulp fibers having a length of the order of 0.1 to 0.45 milliand long, purified 

